P0420 Code only when there is less than half tank of fuel

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by Koich, Jul 3, 2025 at 1:53 PM.

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  1. Koich

    Koich New Member

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    Hello all. This is my first post.
    I searched the forum to look for similar symptoms, but couldn't find any.

    Because the CEL/P0420 only comes on after consuming more than half the tank of fuel, I'm assuming it's something tank-related. I learned that the Gen 2 Prius doesn't have an EGR valve, so assuming it's something else but fuel-related.
    My previous experience with Toyotas with similar problems was fixed with a new gas cap. Maybe I'll try this for the first shot.

    Has anyone experienced anything like this?
     
    #1 Koich, Jul 3, 2025 at 1:53 PM
  2. Brian1954

    Brian1954 Senior Member

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    Does the car have the original catalytic converter? How many miles on the car? Does your engine burn oil?
     
    #2 Brian1954, Jul 4, 2025 at 9:07 AM
  3. frodoz737

    frodoz737 Top Wrench

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    ...and this CAT code goes away by itself when you fill up?
     
    #3 frodoz737, Jul 4, 2025 at 10:14 AM
    Brian1954 and Tombukt2 like this.
  4. Tombukt2

    Tombukt2 Senior Member

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    The catalyst being below threshold is known to the car by it running a self test on the catalyst system at random times . Unknown times as far as I know . So maybe it runs this test right after you fill up with gas.? I have no idea I have nothing to refer to as to how the test works. I don't know of other things being wrong could fool the system into this particular code.
     
    #4 Tombukt2, Jul 4, 2025 at 10:47 AM
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